Data communications by wireless LAN (WLAN) typically according to the IEEE802.11 standards are now widely used. They are used in personal computers (PCs); PC peripherals such as printers, memory apparatuses, broadband rooters, etc.; electronic appliances such as facsimiles, refrigerators, standard-definition televisions (SDTVs), high-definition televisions (HDTVs), digital cameras, digital video-cameras, cell phones, etc.; mobile vehicles such as automobiles, aircrafts, etc., to conduct wireless data communications between wireless signal-transmitting devices, etc.
There are now pluralities of standards of wireless LAN. Among them, IEEE802.11a is adapted to high-speed data communications of 54 Mbps at maximum in a frequency band of 5 GHz, using an OFDM (orthogonal frequency division multiples) modulation system. IEEE802.11b is adapted to high-speed communications of 5.5 Mbps and 11 Mbps by a direct sequence spread spectrum (DSSS) system in an industrial, scientific and medical (ISM) band of 2.4 GHz that can be freely used without license for wireless communications. IEEE802.11g is adapted to high-speed data communications of 54 Mbps at maximum in a 2.4-GHz band like IEEE802.11b, using the OFDM (orthogonal frequency division multiples) modulation system. Explanation will be made below using IEEE802.11b and IEEE802.11g as a first communications system, and IEEE802.11a as a second communications system.
An example of multi-band communications apparatuses using such WLAN is described in JP2003-169008A. As shown in FIG. 32, this multi-band communications apparatus comprises two dual-band antennas capable of transmitting and receiving in two communications systems having different communication frequency bands (IEEE802.11a, IEEE802.11b), two transmitting/receiving means for modulating transmitting data and demodulating received data in each communications system, pluralities of switch means for connecting the antennas to the transmitting/receiving means, and a means for controlling the switch means, so that it can perform diversity receiving.
In this multi-band communications apparatus, frequency scanning is conducted before starting communications to detect receivable frequency channels. To conduct this scanning operation, the antenna ANT1 is connected to a receiving terminal Rx of the transmitting/receiving means of 802.11a, and the antenna ANT2 is connected to a receiving terminal Rx of the transmitting/receiving means of 802.11b, by six single-pole, dual-throw (SPDT) switch means (SW1-SW6). The transmitting/receiving means of 802.11a is scanned in a 5-GHz band, and the transmitting/receiving means of 802.11b is scanned in a 2.4-GHz band, to sense all receivable vacant channels. A signal received by the antenna ANT1 and a signal received by the antenna ANT2 are then compared to activate a system receiving a more desired signal among the two communications systems.
After this scanning operation, the activated transmitting/receiving apparatus is connected to the other antenna, to receive the signal without changing the receiving channel. The two received signals thus obtained are compared, and an antenna receiving a better signal is activated to conduct diversity receiving.
It has been found, however, that such multi-band communications apparatuses suffer from the following problems.
(1) Because disturbance such as phasing, etc. is not taken into consideration in the scanning operation, a communications system receiving the maximum signal is not necessarily selected.
(2) Because many switch means are needed for switching high-frequency signal paths, (a) their control is complicated; (b) the communications loss of the switch means is accumulated, particularly resulting in the deterioration of quality of a high-frequency signal entering through the antenna; and (c) power consumed by the operation of the switch means is not negligible in apparatuses with batteries as power supplies, such as note PCs, cell phones, etc.
(3) High-frequency circuits for WLAN also need filter circuits for removing unnecessary frequency components contained in transmitting signals and received signals, in addition to diversity switches and switch circuits for switching transmitting circuits and receiving circuits. Further, balanced-to-unbalanced converters for converting balanced signal to unbalanced signals, and impedance-converting circuits are needed.
(4) When contained in cell phones or note PCs, or used as network cards of PCMCIA (personal computer memory card international association), it is desired to miniaturize multi-band communications apparatuses.